The study will assess the efficacy of OC000459 200 mg twice daily orally in comparison to placebo when subjects are challenged in the Vienna Challenge Chamber for 6 hours. This is a randomised, double blind, placebo controlled, two way crossover evaluation. There will be a screening period of up to three weeks and a washout period of at least one week between the two treatment periods. There will be a follow up one to three weeks after the last dose of study drug.